A massive police search continued today for an armed gang on the run with up to £40m in cash in what is thought to be Britain's biggest robbery.

The Bank of England said at least £25m had been stolen from a security depot yesterday morning, but sources said the true figure could be as high as £40m, making it the biggest heist in British history.

The Governor of the Bank of England called for a review of security arrangements for the storage of banknotes following the raid on the Securitas cash depot in Tonbridge, Kent.

The raid happened at about 2.15am yesterday when 15 members of staff were threatened and tied up.

The gang, posing as police, abducted the depot manager, his wife and son, and threatened him at gunpoint, forcing him to co-operate.

The heist began when the manager of Securitas' main cash depot south of the Thames was pulled over while driving near Stockbury, Maidstone, at 6.30pm on Tuesday, by what he believed was an unmarked police car.

A man wearing a high-visibility jacket and "police-style" hat got out of the vehicle, which had blue lights in the radiator grille, and spoke to him.

Thinking they were genuine police officers, the manager got into their car where they handcuffed him.

Meanwhile, two more fake policemen visited the manager's wife and young son, told them the manager had been involved in an accident, and took them from their Herne Bay home.

The manager was driven off and the car was later met by a white van.

He was tied up and put inside and taken to an unknown location where he was threatened at gunpoint and told to "co-operate or his family would be at risk".

At about 1am he was taken to the Vale Road security depot where at least six men, some masked and armed with handguns, tied up staff.

The gang loaded the cash into a white lorry before driving off at about 2.15am yesterday.

Police were alerted about an hour later when the shocked but uninjured staff activated an alarm. The manager, his wife and son - who had been taken to an undisclosed location - were also unhurt.

Det Supt Paul Gladstone of Kent Police's serious and organised crime unit said the raid was "clearly a robbery that was planned in detail over time".

The raid at the Northern Bank's Belfast headquarters in December 2004 netted £26.5m, making it the biggest cash theft in British and Irish history at that time.

A Bank of England spokeswoman said: "We have already been reimbursed by Securitas for the initial estimate of £25m and any further sum will be reimbursed to the bank as soon as the amount is known.

"There is no cost at all to the bank or the taxpayer."

She said the bank's governor had asked for a review of the security arrangements for the storage of banknotes.

A Securitas spokesman said an audit to work out exactly how much had been stolen would take place through the night.

The firm's operations director, Tony Benson, said: "The fact no-one has been physically injured should not mask the brutality of this crime and we are all committed to working with the police to catch those responsible and see justice brought to bear."

Det Supt Gladstone added: "It is vitally important we hear from people who may have seen something suspicious or someone acting oddly near the depot in Vale Road, or in any of the locations where this gang operated."